Light emitting diodes (hereinafter referred to as “LEDs”) are semiconductor elements that convert electric energy to light such as ultraviolet light or visible light which is then radiated. An LED lamp comprising a chip of the LED sealed, for example, with a transparent resin is used in various fields. The LED chip is a semiconductor element and thus has a prolonged service life, is highly reliable, and, when used as a light source, can reduce replacement work and the like. Accordingly, the LED chip is becoming widely used as components constituting various display devices such as portable communication equipment, personal computer (PC) peripheral equipment, office automation (OA) apparatuses, domestic electric appliances, signal devices, various switches, and backlight-type display plates.
At the present time, cold-cathode tubes are mainly used in liquid crystal backlights among the above display devices. The cold-cathode tubes contain, for example, mercury and thus are harmful to an environment. On the other hand, as described above, LEDs have many advantages, and, thus, changeover to LEDs has been extensively and intensively studied. The LEDs, however, are inferior in the light intensity difference and brightness uniformity of emitted light to the cold-cathode tubes. Solving these problems is a task necessary to fully apply and develop the LEDs.
There are mainly three types of non-uniformity of emitted light which pose a problem associated with LEDs. The first type of non-uniformity is derived from the fact that LEDs are a small point light source having a typical approximate size of 0.3 mm×0.3 mm×0.25 mm per LED. In use, a plurality of LEDs are generally arrayed on a substrate. No matter how the LEDs are arrayed, a complete line light source or a surface light source cannot be provided. This poses a problem that non-uniformity of brightness occurs due to the presence of a part where a bright light is emitted by LEDs, and a non-light emitting part present between the LED and the other LED.
The second type of non-uniformity is due to a brightness uniformity of light emission by the LED per se. LEDs are usually used in a shape of a rectangular parallelopiped or a shape close to a rectangular parallelopiped. The intensity of light emitted from each face of the rectangular parallelopiped is different from face to face, leading to a difference in brightness of emitted light between directions in which light is taken out from the LED.
The third type of non-uniformity is due to a color uniformity of emitted light. The color uniformity is a problem involved in a combination of LED with a wavelength converting material such as a phosphor material to emitt, for example, white light. For example, an example of a technique for emitting white light, which has been put to practical use up to now, is one disclosed in Japanese Patent Laid-Open No. 315796/2003. The invention described in this publication is directed to a technique in which a white light is provided by combining a blue light emitting LED with a yellow light emitting phosphor, for example, YAG. This technique, however, is disadvantageous in that, when the emitted light is viewed in a certain direction, the emitted light is seen as a yellowish color, and nonuniform yellow and blue colors appear upon projection on a white surface. For this reason, this type of white LED lamps are sometimes called a pseudo white light lamp.
Among the above drawbacks of the LEDs which are disadvantages over cold-cathode tubes, the problem of brightness can be solved to a certain extent by using a number of LEDs. On the other hand, any effective measure for eliminating the problem of non-uniformity of the emitted light has not been proposed, and the problem of non-uniformity of the emitted light has been a big problem.    [Patent document 1] Japanese Patent Laid-Open No. 315796/2003